The design of petroleum crude oil distillation atmospheric towers and reduced crude oil distillation vacuum towers conventionally requires a flash zone into which a mixed phase feed of vapor-liquid is introduced from a feed heater. The flash zone separates an upper rectification zone and a lower stripping zone. The volatile vapor component rises to the bottom rectification tray and the liquid non-volatile residuum falls to the bottom steam stripping trays in which the more volatile components of the descending liquid are stripped and vaporized. The bottoms product is not volatile enough for the bottom draw off stream above the flash zone (gas oil stream) based on vapor-liquid equilibrium but at high feed rates for a tower of given, fixed diameter some undesirable, nonvolatile liquid may be carried up to the gas oil draw by entrainment of the nonvolatile liquid by the vapor, especially when cutting deep into the whole crude or reduced crude.
The undesirable entrained nonvolatile hydrocarbons generally include organo-metallic compounds which poison cracking catalysts. Also, heavy carbonaceous hydrocarbons included in the nonvolatile hydrocarbons limit the yield of high grade distillates or lube oil cuts. If the entrainment of the heavier components can be significantly reduced or eliminated, a significant improvement in the quality of the feed for hydroconversion units, catalytic cracking units and vacuum towers producing valuable gas oil distillates or lube oil distillates both in yield and quality will ensue.
The conventional method for minimizing liquid entrainment in crude distillate towers is to introduce the vapor-liquid feed tangentially into the flash zone through a nozzle into an internal peripheral horn. In a typical peripheral horn design of conventional type, an open-ended, open-bottomed arcuate horn or hood circumferentially abuts the shell wall of the tower. The horn is intended to thrust the liquid component of the heated feed against the tower wall by centrifugal force, causing the liquid to flow downwards by gravity to be collected for distribution across the bottom steam stripping trays; the vapor is intended to flow in a downward direction inside the horn, making a ninety degree to one hundred and eighty degree turn after which it flows up the flash zone toward the bottom rectifying tray with an approximately even velocity pattern. However, recent investigations in the operation of peripheral horns representative of the current state of the art have shown that about 90 to 95 percent of the incoming vapor emerges through the end of the horn, establishing cyclonic patterns of uneven vapor distribution through the flash zone and up through the rectification tray. Further, only a portion of the unflashed nonvolatile hydrocarbon liquid is swept to the inside of the tower shell to flow downward to the stripping trays, leaving the bulk of the remaining liquid to be swept along with the vapor to the flash zone. Because of the uneven velocity pattern striking the bottom rectification or deentrainment tray, through only 20 to 30 percent of its cross-sectional area, high entrainment can occur through the bottom several trays.
Several other types of devices have been employed in the prior art to reduce the entrainment such as demister pads, deentrainment trays, and arrestor plates.
The use of a demister for deentrainment has not been found completely satisfactory for a number of reasons: (1) entrainment in many cases is not significantly reduced; (2) the wire mesh pads have a tendency to plug up with heavy oil and other material such as coke; (3) a wire mesh pad has a tendency to form holes as a result of corrosion.
The installation of various types of deentrainment trays are described in U.S. Pat. No. 3,501,400 to Brody. Deentrainment trays experience problems in that the vapor velocity pattern to the deentrainment tray is not even. The vapor can carry appreciable quantities of entrained liquid with it well up to several fractionating trays above.
U.S. Pat. No. 4,315,815 to Gearhart describes an inlet horn which contains a number of corrugated inlet vanes to separate solid particles from solvent in a solvent recovery operation. The corrugated vanes are designed to cause both solvent and solid to flow down the walls of the stripper. Liquid-vapor separation is not intended or directly achieved in this device through the use of corrugated vanes.
A flash zone peripheral horn or conduit design to achieve vapor liquid deentrainment in crude distillation towers which is commercially available includes a plurality of radial vanes installed vertically in the conduit passageway of the inlet horn. They extend gradually around the horn at an increasing height from the bottom of the horn, approaching a closed conduit end. However, this design does not necessarily assure an even distribution of vaps shown through the vanes.